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D D D D LAPTEV SEA C24 (53 Ma) AMERASIAN BASIN AMERASIAN BASIN GREENLAND GREENLAND ALPHA RIDGE ALPHA RIDGE D D D D LAPTEV SEA C24 (53 Ma) AMERASIAN BASIN AMERASIAN BASIN GREENLAND GREENLAND ALPHA RIDGE ALPHA RIDGE D D D D LAPTEV SEA C24 (53 Ma) AMERASIAN BASIN AMERASIAN BASIN GREENLAND GREENLAND ALPHA RIDGE ALPHA RIDGE 1. DATA 3. CRUSTAL STRUCTURE 4. BREAK-UP SETTING 5. CENOZOIC SEDIMENTATION 1 3 4 5 2 -80 -60 -40 -20 0 20 40 60 80 100 120 mGal FREE-AIR GRAVITY 1 3 4 5 2 Barents sea Amunsen Basin Nansen Basin Gakkel Ridge Yermak Plateau Svalbard Franz Josef Land Severnaya Zemlya m Novaya Ze lya ara s K ea N g G nl n si orwe ian- ree a d Ba n g St. Anna trou h V oni r ug or n t o h Franz-Victoria trough Barents sea Amunsen Basin Nansen Basin Gakkel Ridge Yermak Plateau Svalbard Franz Josef Land Severnaya Zemlya a m a Nov ya Ze ly Kara sea N g Gr nl nd si orwe ian- ee a Ba n n trough St. An a V oni r ug or n t o h anz ic o r g Fr- V t ria t ou h a1 a2 a3 a4 a5 a6 a10 a11 a12 a13 a14 a i2 w 001 b c d e f g g COT BATHYMETRY A series of crustal-scale geotransects illustrating the architecture of the continental margin were constructed using seismic reflection profiles and both inverse and forward gravity modeling. The applied method includes solution of the inverse gravimetric problem with respect to the gravity effect of a thermally differentiated mantle. An iterative grid-based gravity inversion for Moho depth and stretching factors was applied. Two-dimensional gravity inversion for Moho depths was carried out along synthetic profiles taking into consideration the distribution of sedimentary cover from sparse seismic profiles and depth to magnetic source estimates. The crustal transects reveal a narrow and steep continent-ocean transition which is characteristic of sheared more than rifted margins. This may reflect a short-lived phase of shear during breakup prior to the opening of the Eurasia Basin which was initiated at the Paleocene-Eocene transition. The free-air gravity field shows large positive anomalies associated with Plio-Pleistocene glacial fans deposited in front of the Franz-Victoria and St. Anna troughs, which are prominent bathymetric features in the northern Barents-Kara Sea. These sediments were derived from uplifted and eroded areas in the Barents-Kara Sea region. Authors: Alexander Minakov (Department of Earth Science, University of Bergen, ); Jan Inge Faleide (Department of Geosciences, University of Oslo); Vladimir Yu. Glebovsky, (VNIIOkeangeologia, Saint-Petersburg, Russia); Rolf Mjelde (Department of Earth Science, University of Bergen) [email protected] Continental crust Oceanic crust Pre-Cenozoic sediments Cenozoic sediments -35 -30 -25 -20 -15 -10 -5 0 km 0 100 200 300 400 500 600 700 800 Franz Josef Land Western Nansen Basin -35 -30 -25 -20 -15 -10 -5 0 km 0 100 200 300 400 500 600 700 800 Franz Josef Land Central Nansen Basin North Barents Basin 0 100 200 300 400 500 600 700 800 Western Nansen Basin Svalbard Platform Kong Karls Land -35 -30 -25 -20 -15 -10 -5 0 km 1 2 3 -35 -30 -25 -20 -15 -10 -5 0 km 0 100 200 300 400 500 600 700 800 Eastern Nansen Basin St. Anna Basin St. Anna Trough -35 -30 -25 -20 -15 -10 -5 0 km 0 100 200 300 400 500 600 700 800 North Kara Rise Voronin Trough Eastern Nansen Basin 4 5 2 Accumulation (km ) 100 - 200 200 - 500 500 - 1000 > 1000 FVF StAF VF SF BjF a k R i g G k e l d e o e L o m n o s o v R i d g s B i Amund en as n o B n Makar v asi n Ca ada n Basi Knipovi h Rid c ge Mohn Ridge a nt n r S i A na t . Fran to z Vic ria tr. roni t Vo n r. Nansen Basin ende eev idg M l R e lp a R ge A h id k Yerma P t la eau Svalbard Gre na d eln Ele m re s e an Isl d Franz f Jose L nd a Se e n ya vra m ya Ze l Novaya Zemlya or M r is p Jesu ise R N rs trait aeS Lat p ev Sea Eurasian Basin Labrador Sea w ga Nor ein Ge l d r en an B n asi Barents Sea Kar a a Se Amerasian Basin Bjørnaya tr. The northern Barents Sea continental margin has remained the least investigated province of the Barents Sea because of very limited seismic data due to a largely permanent ice cover. An understanding of the structure and the evolution of the continental margin is essential to figure out the history of geological development and the hydrocarbon potential of the northern Barents Sea region. ABSTRACT 2. METHOD grain density -3 Density (g/cm ) Depth (km) -1 Compaction: 0.65 km Porosity at sea bottom: 0.52 -3 Grain density: 2.7 g/cm -3 Density at sea bottom: 1.82 g/cm ns f we at De ity rom ll d a -300 -240 -180 -120 -60 0 60 120 180 240 300 360 420 mGal -80 -60 -40 -20 0 20 0 4 60 80 10 0 ° 30 3° 0 60° 60° 90° 90° 80° 80° 5 8° 85° BOUGUER GRAVITY 30˚ 30˚ 60˚ 60˚ 90˚ 90˚ 80˚ 0 8˚ 85˚ 5 8˚ 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 mln. yr. CRUSTAL AGE -300 -240 -180 -120 -60 0 60 120 180 240 300 360 420 mGal 0 0 2 0 4 3° 0 30° 60° 60° 90° 0 9° 80° 80° 85° 85° THERMAL GRAVITY CORRECTION -300 -240 -180 -120 -60 0 60 120 180 240 300 360 420 mGal 0 -1 0 0 -8 -60 4 -0 -20 0 20 40 60 80 100 120 140 3° 0 30° 60° 60° 90° 0 9° 80° 80° 85° 85° MANTLE RESIDUAL ANOMALY 8 12 16 20 24 28 32 36 40 km 12 14 16 18 20 22 24 26 28 30 2 3 2 3 32 34 34 34 3° 0 30° 60° 60° 0 9° 0 9° 80° 80° 5 8° 85° 1 2 3 4 5 MOHO DEPTH AND CRUSTAL THICKNESS ESTIMATIONS STRETCHING FACTORS . 11 1.1 1.1 1.3 1.7 5 2. 9 2. 3.3 3.5 3.5 3.7 . 41 30˚ 30˚ 60˚ 60˚ 9˚ 0 9˚ 0 80˚ 0 8˚ 85˚ 5 8˚ 1.0 1.5 2.0 2.5 3.0 3.5 4.0 1000 0 100 200 300 400 500 600 700 800 -35 -30 -25 -20 -15 -10 -5 0 km 0 100 200 300 400 500 600 700 800 -300 -200 -100 0 100 200 300 Mantle residual anomaly (mGal) Distance (km) 300 Ma 58 Ma < 54 Ma 3 0 100 200 300 400 500 600 700 800 -300 -200 -100 0 100 200 300 Mantle residual anomaly (mGal) Distance (km) 0 100 200 300 400 500 600 700 800 -35 -30 -25 -20 -15 -10 -5 0 km 300 Ma 58 Ma < 54 Ma 1 input residual gravity anomaly calculated anomaly from inverted Moho calculated anomaly from isostatic Moho depth to magnetic sources estimates Moho inverted from gravity Moho according to local isostasy Moho according to regional isostasy theoretical subsidence corrected for sedimentary load -4 -6 -8 -10 -12 -14 -16 -18 -20 14 16 18 20 22 24 26 0.8754 3.322 14.82 0.0681 0.1595 0.3697 0.017 0.0193 0.0346 Dr=0 .4 g/cm 3 Dr=0 .6 g/cm 3 Dr=0 .5 g/cm 3 RMS increases Thickening of the continental crust Oceanic Moho depth estimates (km) Downward continuation depth (km) RMS increases GRAVITY ISOSTASY 2 2 1 > Crustal density 1 2 1 SENSITIVITY TO CRUSTAL DENSITY CHANGE AND DOWNWARD CONTINUATION DEPTH 3D GRAVITY INVERSION INCORPORATING LITHOSPHERE THERMAL GRAVITY ANOMALY 2D GRAVITY INVERSION INCORPORATING LITHOSPHERE THERMAL GRAVITY CORRECTION, EXPONENTIAL DENSITY-DEPTH FUNCTION IN SEDIMENTS AND ISOSTATIC CONSTRAINTS -2 0 0 -100 -100 -100 0 -1 0 0 -1 0 -100 -100 -100 0 -1 0 -100 -100 -100 -100 1 - 00 10 -0 -100 - 00 1 -1 0 0 -100 -100 -50 -50 -50 -50 -50 -50 50 - -50 -50 -50 -50 5 -0 -50 5 -0 -50 -50 -50 -50 -50 -50 0 -5 -50 -50 -0 5 -50 -50 0 -5 -50 -50 5 -0 -50 0 -5 -50 -50 -50 -50 -50 -50 -50 -50 -50 -50 -50 -50 -50 -25 -25 -25 -25 -25 2 -5 -25 5 -2 5 -2 2 -5 -25 -25 -25 -5 2 -5 2 -25 -25 -25 -25 -25 2 -5 -25 -25 5 -2 -25 5 -2 -25 2 -5 -25 -25 5 -2 -25 -25 -25 -25 5 -2 -25 -25 -25 -25 -25 2 -5 -25 2 -5 -25 -25 2 -5 -25 -25 -25 -5 2 -25 -25 -25 25 - -25 2 -5 -25 -25 -25 -25 25 - -25 5 -2 -25 -25 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 5 2 5 2 5 2 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 5 2 25 25 5 2 25 50 50 50 50 50 50 50 50 0 5 50 50 50 50 50 50 50 0 5 50 50 50 50 0 5 50 0 5 50 50 50 50 50 50 50 50 0 5 50 50 50 100 100 100 0 10 0 10 100 10 0 0 10 100 10 0 0 10 100 100 0 10 100 100 100 100 100 100 0 10 100 100 0 10 0 10 100 100 100 200 200 200 200 200 20 0 200 200 0 20 200 0 40 400 0° 0° 30° 6° 0 60° 90° 120° ° 120 -3 ° 0 ° 70 70° 7° 5 75° 0° 8 80° 85° 85° 24n (53) 24n (53) 20n (43) 20n (43) 18n (40) 18n (40) 13n (33) 13n (33) 6n (20) 6n (20) 5n (11) 5n (11) 2an (3.5) 2an (3.5) 1 2 3 4 5 MAGNETIC ANOMALIES 0 200 400 600 800 nT S N SOME CONSTRAINTS FOR SCENARIOS OF THE NORTHERN BARENTS SEA MARGIN EVOLUTION -The sedimentary cover in the Nansen Basin was influenced by three large fans (Franz Victoria, Saint Anna and Voronin) which are well pronounced in the modern sea floor topography and free air gravity; -The sedimentary thickness in the Nansen Basin was estimated as about 2-4 km; -It is believed that the one third of that thickness was created during Late Cenozoic due to Pliocene-Pleistocene uplift and glacial erosion events; -It is predicted 3-4 km stronger lithified sediments below the upper unit in front of St. Anna Trough. - The Northern Barents/Kara continental margin is characterized by narrow continent ocean transition (50 - 80 km); - Crustal thickness increases from the inner part of the northern Barents Sea area to the periphery constituting 20-27 km and 30 - 34 km of the crystalline crust, respectively; - North Kara Sea area was modeled with 30 km of the crystalline crust throughout; - The oceanic crust is thinned to around 5 km in the Nansen Basin; - Sediment thickness decreases from 6-13 km to 1-4 km from the inner shelf towards the continent ocean transition; It is believed that the most probable mechanism for the formation of the narrow northern Barents/Kara Sea margin was a short-lived episode of shear/transtension connected to Labrador Sea/Baffin Bay spreading system 56-69 Ma
